The present research work was carried out in the Yurimaguas agroecological farm at 8.5 km from the Yurimaguas - Tarapoto highway, which provided a scope on the "Dosage of nitrogen fertilizer in forage yield of Pennisetum sp accession" King grass morado "in Yurimaguas , Peru - 2016 ". Five doses of nitrogen fertilizer were used (46% urea). The evaluations were made on the eighth week after sowing with vegetative seeds, in plots of 3.6 m2 of area and 141.1 m2 of total area, established in an Ultisol soil. The statistical design used was a Design of Block Completely Random (DBCA), with five treatments and four repetitions, using the statistical package of Inforstat, the treatments under study were: T0 (0 t / ha of nitrogen fertilizer), T1 (50 t / ha of nitrogen fertilizer), T2 (100 t / ha of nitrogen fertilizer), T3 (150 t / ha of nitrogen fertilizer) and T4 (200 t / ha of nitrogen fertilizer). In the present investigation an average plant height equal to 144.45 cm, green matter of 4.32 kg / m2 and dry matter of 0.86 kg / m2 was obtained with the T4 treatment (200 N kg / ha / ha), In relation to the average of green matter (kg / m2), it shows that as the nitrogen dose increases, the increase in yield decreases. This decrease in yield is due to Mitscherlich's theory of law